Method for hermetically sealing optical fiber introducing section and hermetically sealed structure

ABSTRACT

A conventional optical fiber not coated with a metal layer is provided. A solder layer is filled into between a case, for an optical module wherein an optical element is optically coupled with the optical fiber within the case, and a cover conforming to the shape of the case, thereby simply conducting hermetical sealing. Thereafter, a resin, such as a silicone resin, is potted in the optical fiber introducing section. The two-layer hermetical sealing can realize a method for hermetically sealing an optical fiber introducing section which can maintain airtightness high enough to ensure long-term reliability of an optical module. This can facilitate the hermetical sealing of an optical device particularly in its optical fiber introducing section.

RELATED APPLICATION

This a division of application Ser. No. 09/061,518, filed Apr. 16, 1998,now U.S. Pat. No. 6,074,104.

FIELD OF THE INVENTION

The present invention relates to a method for hermetically sealing anoptical device, and particularly to a hermetically sealing method and ahermetically sealed structure in an optical fiber introducing section ofan optical module having such a structure that an optical fiber strandis used in an introducing terminal.

BACKGROUND OF THE INVENTION

An optical module is a structure for optically coupling an opticalelement, such as a semiconductor laser element or a photodetector, withan optical fiber. The optical element should be protected againstmoisture from the viewpoints of stabilization of properties and highreliability. In general, these optical elements are housed in a case,and the case is hermetically sealed to protect the optical elementagainst moisture outside the case.

Methods for hermetically sealing the case are roughly classifiedaccording to the structure of the optical module into two methods, thatis, a method wherein a window permeable to light is provided in the caseand an optical element provided within the case is optically coupledwith an optical fiber outside the case through the window and a methodwherein an optical element and an optical fiber are coupled with eachother within the case. In recent years, the latter method is mainlyadopted from the viewpoint of reducing the size of the optical device.

In a structure provided by the latter method, that is, in a structurewherein an optical element and an optical fiber are coupled with eachother within the case, hermetical sealing should be performed betweenthe optical fiber and the case.

A conventional hermetically sealed structure in an optical fiberintroducing section is proposed, for example, in Japanese PatentLaid-Open No. 92334/1995. The hermetically sealed structure in anoptical fiber introducing section described in this publicationcomprises: a case; a silicon substrate, with an optical element mountedthereon, fixed within the case; and an optical fiber disposed at aposition where the optical fiber is optically coupled with the opticalelement, and fixed to the silicon substrate.

The optical fiber used in this case is a metal-coated fiber formed bycoating a metal layer on the surface of an optical fiber strand, with aresin coating removed therefrom, by plating, sputtering or other method.The coating metal is a material having good solderability, such asnickel or gold. The metal-coated fiber is inserted into between the caseand the cover, followed by soldering. As a result, the solder joins thecase to the cover and, at the same time, joins the metal layer on thesurface of the metal-coated fiber, thus hermetically sealing theinterior of the case. Another conventional hermetically sealed structurein an optical fiber introducing section is described, for example, inJapanese Patent Laid-Open No. 198973/1995, wherein a low-melting glassis used.

In this hermetically sealed structure, a quartz optical fiber isprovided without coating of a metal layer, and an optical element andthe optical fiber are optically coupled with each other, followed byfixation of the optical fiber onto a silicon substrate.

The optical fiber is then inserted into between a case and a cover, alow-melting glass is provided in a portion, between the case and thecover, in its whole circumference, and the low-melting glass is thenheat-melted to hermetically seal the interior of the case.

Still another conventional hermetically sealed structure in an opticalfiber introducing section is such that the circumference of an opticalfiber is sealed with a resin having a hermetically sealing property. Inthis hermetically sealed structure in an optical fiber introducingsection, an optical fiber is provided without coating of a metal layer,an optical element and the optical fiber are optically coupled with eachother, followed by fixation of the optical fiber onto a siliconsubstrate.

The optical fiber is then inserted into between a case and a cover, theresin is filled into a portion between the case and the cover, in itswhole circumference, and the resin is then cured to hermetically sealthe interior of the case.

The conventional hermetically sealed structures in an optical fiberintroducing section have the following respective problems.

(1) The hermetically sealed structure in an optical fiber introducingsection using a metal-coated fiber suffers from a problem that the costof coating the metal layer onto the optical fiber is high.

An additional problem involved in this structure is that selectivecoating of the metal layer onto the optical fiber introducing sectionalone lowers the mass productivity.

Further, when the optical fiber is disposed on a V-groove in order toconduct positioning of the optical element and the optical fiberrelative to each other, a variation in the thickness of the metal layerresulted in varied outer diameter of the optical fiber,disadvantageously deteriorating the positioning accuracy.

(2) For the hermetically sealed structure in an optical fiberintroducing section using a low-melting glass, since the melting pointof the low-melting glass is generally high and 430° C. or above, at thetime of sealing, the low-melting glass is likely to cause damage to theoptical element fixed portion within the case and the optical fiberprovided with a protective coating.

(3) The hermetically sealed structure in an optical fiber introducingsection using a resin has problems including that the airtightness isgreatly influenced by resin coating methods, seal path length, curingconditions and the like, making it impossible to stably conduct thesealing work, the hermetically sealing level of the resin per se isabout 10⁻⁵ atm.cc/sec and unsatisfactory for use in optical modules, thereliability under severe environmental conditions such as hightemperature and high humidity is low, and the sealed structure is notresistant to heat.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide ahermetically sealing method and a hermetically sealed structure in anoptical fiber introducing section which can eliminate the need to use anexpensive metal-coated fiber, can use a solder material having a meltingpoint below that of a low-melting glass, and enables airtightness highenough to ensure long-term reliability of an optical module to bemaintained even under severe environmental conditions.

According to the first feature of the invention, a method forhermetically sealing an optical fiber introducing section in an opticaldevice comprises an optical fiber, an optical element optically coupledwith the optical fiber, a case for housing therein the optical element,and a cover for sealing the case,

the optical fiber propagating light between the interior and theexterior of the case,

said method comprising the steps of: melting and filling a solder intobetween the optical fiber and the case; hardening the filled solder; andfilling a resin into between the optical fiber and the solder and curingthe resin.

According to the second feature of the invention, a hermetically sealedstructure of an optical device, comprises:

an optical fiber;

an optical element optically coupled with the optical fiber;

a case for housing therein the optical element; and

a cover for sealing the case,

the optical fiber propagating light between the interior and theexterior of the case,

wherein the optical fiber is fixed to the case, in its section at whichthe optical fiber is introduced into the case, with the aid of ahermetically sealing solder and a resin is filled so as to cover atleast a connection between the optical fiber and the solder.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail in conjunction withappended drawings, wherein:

FIG. 1 is a cross-sectional view showing a conventional hermeticallysealed structure in an optical fiber introducing section;

FIG. 2 is a cross-sectional view showing another conventionalhermetically sealed structure in an optical fiber introducing section;

FIG. 3 is a cross-sectional view showing still another conventionalhermetically sealed structure in an optical fiber introducing section;

FIG. 4 is a perspective view showing a first preferred embodiment of thepresent invention;

FIG. 5 is an explanatory cross-sectional view showing a hermeticallysealed structure according to a first preferred embodiment of thepresent invention;

FIG. 6 is a perspective view showing a second preferred embodiment ofthe present invention; and

FIG. 7 is a perspective view showing a third preferred embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before describing the hermetically sealed structure in an optical fiberintroducing section in a preferred embodiment according to the presentinvention, the aforementioned conventional hermetically sealedstructures in an optical fiber introducing section will be explained inconjunction with FIGS. 1 to 3.

A first conventional method for hermetically sealing an optical fiberintroducing section is proposed, for example, in Japanese PatentLaid-Open No. 92334/1995. This method is shown in FIG. 1. Specifically,FIG. 1 is a longitudinal sectional view illustrating the method forhermetically sealing an optical fiber introducing section described inJapanese Patent Laid-Open No. 92334/1995. The first conventional methodwill be explained with reference to FIG. 1.

In FIG. 1, a silicon substrate 1 with an optical element 19 mountedthereon is fixed within a case 5. An optical fiber 15 is then disposed,at a position wherein the optical fiber 15 is to be optically coupledwith the optical element 19, and fixed to the silicon substrate 1.

The optical fiber used in this case is a metal-coated fiber 15 formed bycoating a metal layer 16 on the surface of an optical fiber strand, witha resin coating removed therefrom, by plating, sputtering or othermethod. The coating metal is a material having good solderability, suchas nickel or gold.

The metal-coated fiber 15 is inserted into between the case 5 and thecover 8, followed by soldering. As a result, the solder 9 joins the case5 to the cover 8 and, at the same time, joins the metal layer 16 on thesurface of the metal-coated fiber 15, thus hermetically sealing theinterior of the case.

A second conventional technique is described, for example, in JapanesePatent Laid-Open No. 198973/1995. This publication discloses ahermetically sealed structure in an optical fiber introducing section,wherein a low-melting glass is used. FIG. 2 is a longitudinal sectionalview showing the hermetically sealed structure in an optical fiberintroducing section according to the second conventional technique. Thesecond conventional technique will be explained with reference to FIG.2.

In FIG. 2, a quartz optical fiber is provided without coating of a metallayer, and, as with the first conventional technique, an optical element19 and the optical fiber 7 are optically coupled with each other,followed by fixation of the optical fiber 7 onto a silicon substrate 1.

The optical fiber 7 is then inserted into between a case 5 and a cover8, a low-melting glass 17 is provided in a portion, between the case 5and the cover 8, in its whole circumference, and the low-melting glassis then heat-melted to hermetically seal the interior of the case.

A third prior art technique is a hermetically sealed structure in anoptical fiber introducing section. This structure is such that thecircumference of an optical fiber is sealed with a resin having ahermetically sealing property. FIG. 3 shows a longitudinal sectionalview of the hermetically sealed structure in an optical fiberintroducing section according to the third conventional technique. Thethird conventional technique will be explained with reference to FIG. 3.

In FIG. 3, an optical fiber is provided without coating of a metallayer, and, as with the first conventional technique, an optical element19 and the optical fiber 7 are optically coupled with each other,followed by fixation of the optical fiber 7 onto a silicon substrate 1.

The optical fiber 7 is then inserted into between a case 5 and a cover8, a resin 12 is filled into a portion between the case 5 and the cover8, in its whole circumference, and the resin 12 is then cured tohermetically seal the interior of the case.

Next, preferred embodiments of the present invention will be described.The method for hermetically sealing an optical fiber introducing sectionaccording to the present invention, in its preferred embodiment, ischaracterized in that a conventional quartz optical fiber withoutcoating of a metal layer is provided, an optical element and the opticalfiber are optically coupled with each other within a case, a solder isfilled into between the case of an optical module and a cover conformingto the shape of the case, thereby simply conducting hermetical sealing,and a resin, such as an adhesive, is filled into the optical fiberintroducing section and then cured.

According to the preferred embodiment of the present invention, the caseand the cover are hermetically sealed with a solder, and, in the opticalfiber introducing section, the circumference of the optical fiber issubstantially filled with the solder. Since the optical fiber per se ismade of quartz glass, it is not subjected to alloyed junction by thesolder, resulting in the creation of minute gaps at boundaries betweenthe solder, which has been solidified after melting, and the opticalfiber. Filling of the resin into between the optical fiber and thesolder followed by curing of the resin can realize a high degree ofairtightness of the optical device.

Thus, according to the preferred embodiment of the present invention,use of a conventional optical fiber without coating of a metal layer inthe hermetical sealing realizes hermetical sealing of an optical fiberintroducing section at a low cost.

Further, the variation in outer diameter of the optical fiber is sosmall that provision of a V-groove within the case enables thepositioning of the optical fiber to be performed with a high accuracy.In addition, the attainable airtightness is high enough to ensurelong-term reliability of an optical module, preferably not more than10⁻⁷ atm.cc/sec.

FIG. 4 is a perspective view showing assembly of a first preferredembodiment of the hermetically sealing method according to the presentinvention. In FIG. 4, a silicon substrate 1 is one prepared by forming anecessary electrode pattern on both sides of a silicon wafer byphotolithography and conducting chipping by dicing.

In the silicon substrate 1, a V-groove 2 is formed simultaneously withthe formation of an electrode pattern. The groove 2 is formed byanisotropic etching. Since the shape of the groove 2 can be madeconstant, the positioning of the optical fiber can be performed with asubmicron accuracy.

A semiconductor laser 4 is fixed, with a high-temperature solder, suchas AuSn, to the electrode 3 formed by patterning on the top surface ofthe silicon substrate 1.

The silicon substrate 1 is fixed, with high-temperature solder, such asAuSn, to the cavity section of the case 5. At that time, positioning andfixing are carried out so that the center of the V-groove 2 is locatedat a given position relative to a fiber introducing section 6 providedin the case 5.

The case 5 is preferably a metallic case made of Kovar or the like, aceramic package or the like from the viewpoints of airtightness,soldability, and cost.

The optical fiber 7 is highly acculately positioned relative to andoptically coupled with the semiconductor laser 4 through the V-groove 2,and fixed to the V-groove 2 with a high-temperature solder, such asAuSn, or a heat-resistant adhesive, such as an epoxy adhesive.

A thin metal layer of nickel or gold is provided by plating on the cover8, a hermetically sealing face 10, and the fiber introducing section 6in consideration of solderability.

A solder material having a melting point, which has no thermal influenceon the assembly section within the case, is used for the solder 9.Solder materials usable herein include Sn-Pb solder, Sn-Sb solder, Sn-Insolder, and Au-Sn solder. The solder material is selected by taking intoconsideration the thermal influence on the interior of the case,temperature conditions under which the structure after hermeticalsealing can withstand and the like.

In particular, when an Au-Sn solder is used for joining componentswithin the case with an SnPb eutectic solder being used for the fixationof the case to a print board, an Sn-Sb solder or an Sn-In solder isselected as the solder for hermetical sealing so that the melting pointof the solder for hermetical sealing is Au-Sn >solder for hermeticalsealing >Sn-Pb.

The cover 8 and the solder 9 are positioned so as to conform to theshape of the case in its hermetically sealing face 10. The solder 9 ispreviously formed in the form of a square washer or the like so that itcan be easily fed in the case 5.

Thereafter, the assembly is heated to the melting point of the solder 9,thereby fixing the cover 8, the case 5, and the optical fiber 7 with theaid of the solder 9.

At that time, the circumference of the optical fiber 7 is filled with asolder layer 11 (see FIG. 5). Since, however, the solder layer 11 is notsubjected to alloyed junction with the optical fiber 7 by the solder,reliable airtightness cannot be ensured in this stage.

A resin 12 is then filled on the surface of the optical fiber 7 and thesolder layer 11 (see FIG. 5) and then cured, thereby conductinghermetical sealing. FIG. 5 is a longitudinal sectional view of ahermetically sealed structure completed through the above stepsaccording to this preferred embodiment.

In this case, a resin having a viscosity of not more than 70 Pa.s isused as the resin 12 so that the resin can be easily filled into minutegaps.

In particular, when a cold curing silicone rubber is applied as theresin 12, good airtightness can be ensured over a wide temperature rangebecause the silicone rubber has a cold/heat resistance of −40° C. to200° C.

When good airtightness at a usual environmental temperature iscontemplated, an epoxy resin or a UV (ultraviolet) resin may also beused as the resin 12.

Filling of the resin followed by curing of the resin permits minute gapsat boundaries between the solder layer 11 and the optical fiber 7 to befilled with the resin, which can provide airtightness high enough toensure the reliability of the optical device, that is, an airtightnessof not more than 10⁻⁷ atm.cc/sec.

In general, in the case of hermetical sealing with a resin alone, it isdifficult to provide satisfactory airtightness due to low hermeticalsealing level of the resin per se, uneven coating, unsatisfactorysealing area and the like. However, use of the resin for filling intominute gaps between the optical fiber and the solder layer according thepresent preferred embodiment of the present invention can greatlyimprove the airtightness of the whole case.

The second preferred embodiment of the present invention will beexplained in detail with reference to FIG. 6. FIG. 6 is a perspectiveview showing assembling of a hermetically sealed structure according tothe second preferred embodiment of the present invention. In FIG. 6, inthe present preferred embodiment, as with the first preferredembodiment, after a silicon substrate 1 with a semiconductor laser 4mounted thereon is fixed within the case 5, an optical fiber 7 ismounted. A solder plating 13 is previously provided on a cover 8 and acase 5.

After the cover 8 and the case 5 are disposed at respectivepredetermined positions, the temperature is raised to the remeltingtemperature of the solder plating 13, thereby fixing the cover 8, thecase 5, and the fiber 7 with the solder. Thereafter, hermetical sealingwith a resin 12 is carried out.

Next, the third preferred embodiment of the present invention will beexplained in detail with reference to FIG. 7. FIG. 7 is a perspectiveview showing assembling of a hermetically sealed structure according tothe third preferred embodiment of the present invention. In FIG. 7, inthe present preferred embodiment, as with the first preferredembodiment, a silicon substrate 1 with a semiconductor laser 4 mountedthereon is fixed within the case 5.

In the case 5, a cavity hole 20, passing through the case, rather thanthe rectangular groove is provided as the fiber introducing section.

At the time of mount of an optical fiber 7, the optical fiber 7 isinserted from the transverse direction and mounted and fixed onto asilicon substrate 1. The fixation of the cover 8 and the case 5 with asolder may be carried out in the same manner as described above inconnection with the first preferred embodiment. In this case, the solderis also filled into the cavity hole 20. Thereafter, hermetical sealingwith a resin 12 is carried out.

Joining between the cover 8 and the case 5 may be carried out byairtight welding, such as seam welding, rather than the soldering. Inthis case, the filling of the solder into the cavity hole 20 is carriedout as a separate step, followed by hermetical sealing with a resin 12.

As described above, according to the method for hermetically sealing anoptical fiber introducing section according to the present invention, aconventional quartz optical fiber is used without coating of a metallayer onto the optical fiber, advantageously enabling a hermeticallysealed structure to be provided at a low cost.

Further, according to the present invention, since hermetical sealing iscarried out using a solder having a melting point below that of alow-melting glass, a thermal influence on the optical element fixingsection within the case and the protective coating of the optical fibercan be avoided.

Furthermore, filling of a solder around the optical fiber and filling ofa resin into minute gaps at boundaries between the optical fiber and thesolder followed by curing of the resin can simply realize a hermeticallysealed structure having higher airtightness and reliability thanhermetical sealing with the solder alone or the resin alone.

The invention has been described in detail with particular reference topreferred embodiments, but it will be understood that variations andmodifications can be effected within the scope of the present inventionas set forth in the appended claims.

What is claimed is:
 1. A method for hermetically sealing a space throughwhich an uncovered optical fiber extends into a case which houses anoptical element which is optically coupled to the optical fiber, saidmethod comprising: filling at least a portion of the space with ahermetically sealing solder so as to substantially hermetically seal thespace while leaving minute gaps at the boundaries between the solder andthe uncovered optical fiber; and covering at least a portion of thesolder with a hermetically sealing resin so as to close at least some ofthe minute gaps.
 2. A method according to claim 1, further including thestep of placing the optical fiber in a groove located in the case foraligning an end of the optical fiber with the optical element so as tooptically couple the optical fiber to the optical element.
 3. A methodaccording to claim 2, wherein the optical element is mounted on asubstrate located in the case and the groove is formed in the substrate.4. A method according to claim 3, wherein the substrate is a siliconsubstrate.
 5. A method according to claim 1, wherein the optical fiberis made of quartz.
 6. A method according to claim 1, wherein the solderis formed between the optical fiber and a surrounding portion of thecase and wherein the resin in located between the optical fiber and anouter surface of the case.
 7. A method according to claim 6, wherein thecase includes a cover and wherein the surrounding portion of the case isa portion of the cover.
 8. A method according to claim 1, wherein thesolder does not contain any flux component.
 9. A method according toclaim 8, wherein the resin is an epoxy resin.
 10. A method according toclaim 8, wherein the resin is a silicon resin.
 11. A method according toclaim 8, wherein the resin is a UV resin.
 12. A method according toclaim 1, wherein the resin is an epoxy resin.
 13. A method according toclaim 1, wherein the resin is a silicon resin.
 14. A method according toclaim 1, wherein the resin is a UV resin.
 15. A hermetically sealedstructure, comprising: a case; an optical element located in the case;an uncovered optical fiber extending from a position outside of saidcase to a position inside of said case, the optical fiber beingoptically coupled to the optical element, the optical fiber extendinginto the case through an entry passage formed in the case; ahermetically sealing solder filling at least a portion of the entrypassage so as to substantially hermetically seal the space while leavingminute gaps at the boundaries between the solder and the uncoveredoptical fiber; and a hermetically sealing resin covering at least aportion of the solder so as to close at least some of the minute gaps.16. A hermetically sealed structure according to claim 15, furtherincluding a groove located in the case for aligning an end of theoptical fiber with the optical element so as to optically couple theoptical fiber to the optical element, the optical fiber being located inthe groove.
 17. A hermetically sealed structure according to claim 16,wherein the optical element is mounted on a substrate located in thehousing and the groove is formed in the substrate.
 18. A hermeticallysealed structure according to claim 17, wherein the substrate is asilicon substrate.
 19. A hermetically sealed structure according toclaim 15, wherein the optical fiber is made of quartz.
 20. Ahermetically sealed structure according to claim 15, wherein the solderis formed between the optical fiber and a surrounding portion of thecase and wherein the resin in located between the optical fiber and anouter surface of the case.
 21. A hermetically sealed structure accordingto claim 20, wherein the case includes a cover and wherein thesurrounding portion of the case is a portion of the cover.
 22. Ahermetically sealed structure according to claim 15, wherein the solderdoes not contain any flux component.
 23. A hermetically sealed structureaccording to claim 22, wherein the resin is an epoxy resin.
 24. A methodaccording to claim 22, wherein the resin is a silicon resin.
 25. Amethod according to claim 22, wherein the resin is a UV resin.
 26. Ahermetically sealed structure according to claim 15, wherein the resinis an epoxy resin.
 27. A method according to claim 15, wherein the resinis a silicon resin.
 28. A method according to claim 15, wherein theresin is a UV resin.